Backlight adjustment tool similar to xbacklight but with cubic scale.

This is intended for backlight devices with large raw adjustment range
(0..900, intel_backlight nodes in /sys/class/backlight) and linear
brightness response. The tool switches backlight brightness between
reasonably small number (20 by default) of positions chosen to compensate
for non-linearity in human perception, making the steps between positions
visually more or less equal.

Not for devices with small ranges like 0..15 (acpi_backlight and such).
Use xbacklight with those instead.


See the following on why the scale is chosen to be cubic:
https://en.wikipedia.org/wiki/Lightness
http://warrenmars.com/photography/technical/resolution/human_perception.htm
